The field of art to which this invention pertains is catalysts for the high temperature shift reaction.
Hydrogen production from carbon monoxide and steam has for decades been one of the most important processes of the chemical industry. This reaction EQU CO+H.sub.2 O.revreaction.H.sub.2 +CO.sub.2
is commonly known as the water gas shift reaction which term was derived from 19th centruy generation of water gas which was manufactured by passing steam over hot coal to produce a mixture of carbon monoxide and hydrogen. The conversion of carbon monoxide by this reaction is favored by lower temperatures and the reaction is slightly exothermic.
While lower tempertures favor more complete carbon monoxide conversion, higher temperatures allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature shift reaction unit for bulk carbon monoxide conversion and heat recovery, and a low temperature shift reaction unit for final carbon monoxide conversion.
The high temperature shift reaction is generally conducted at a temperature range of about 600.degree.-950.degree. F. using iron-chrome catalysts. The low temperature shift reaction is generally conducted at a temperature range of about 350.degree.-550.degree. F. using copper-zinc catalysts.
U.S. Pat. No. 4,540,563 describes shift reaction catalysts made of iron oxide and at least one other metal oxide which is difficult to reduce to metal.
High temperature shift reaction catalysts made of iron oxide and chromium oxide modified with magnesium oxide are disclosed in U.S. Pat. No. 4,598,062.
In U.S. Pat. No. 4,503,162, shift reaction catalysts made by coprecipitating iron, aluminum and copper salts followed by drying and calcining are described.
According to French Pat. No. 2,554,433, shift reaction catalysts are made by grinding together lithium carbonate, copper oxide, iron oxide and iron nitrate, drying the powder and calcining it. In French Pat. No. 2,553,399, catalysts are made by a similar procedure form manganese carbonate, copper nitrate, ferric oxide and chromic nitrate.
Prasad and Sarma, Fertilizer Technology, Vol. 17, (1 and 2), 62-65 (1980) describe the effect of copper and potassium on the activity of iron-oxcide-chrome oxide shift reaction catalysts. These catalysts, which contain ferric oxide in admixture with chromic oxide in the amount of 7-8% and copper oxide in the amount of 0.5-2%, were made from ferric oxide kneaded with chromic acid and copper acetate, followed by drying and sintering. The copper containing catalysts were not as active in the high temperature shift reaction at a steam to gas ratio of 3:1 as catalysts which contained no copper.
In an effort to save energy in the production of hydrogen and ammonia, plants sometimes attempt to operate with lower amounts of steam added to the steam hydrocarbon reforming unit. The consequence of this low steam/carbon ratio (S/C) is that the high temperature shift unit will be forced to operate at lower steam to gas ratios (S/G) and higher carbon monoxide/carbon dioxide ratios. The combination of these factors in the high temperature shift reaction using iron-chromium catalysts can lead to hydrocarbon by-product formation via the Fischer-Tropsch reaction.